Enabling broad impact utilizing the tremendously varied properties available in complex oxide thin films requires a means of integration on technologically relevant substrates, specifically mainstream semiconductors such as silicon. Many such devices require substrate metallization to form electrical contacts, and the most widely and technologically relevant is platinum-coated silicon. Platinized silicon offers great flexibility in being chemically inert in contact with many oxides, compatible with moderate processing temperatures in oxidizing, inert, or moderately-reducing atmosphere (thus requiring little or no process alteration for integration with any of a variety of complex oxide film systems), and is relatively inexpensive when used in thin layers. As such, while optimization of electrode (and/or substrate)-film interactions is emphasized for complex oxides deposited on base metals, oxide electrodes, and exotic substrates, the electrode/substrate interface is too often simply ignored or, at best, considered entirely inert, passive observers in the integration process in the vast majority of work on platinized silicon.
In spite of the extensive earlier efforts toward preparing high-quality, temperature-stable platinized silicon substrates for ferroelectric memory integration, several issues remain that are detrimental to the deposited oxide properties. The most pervasive of these are thermophysical instabilities at temperatures in excess of 700° C. including delamination and hillocking as well as diffusion of adhesion layers through the platinum; these defects often cause degraded performance and can result in inoperable devices. See H. N. Al-Shareef et al., J. Mater. Res. 12, 347 (1997). Additionally, the (presumed) 700° C. limit for thermal processing of films on platinum-coated silicon substrates has been highlighted as the primary source of the chasm between the (poor) measured properties of refractory oxides such as BaTiO3 films on silicon wafers and the bulk-like behavior that has been achieved by processing these films at higher temperatures on other substrates. See S. M. Aygun et al., J. Appl. Phys. 109, 034108 (2011); and J. F. Ihlefeld et al., J. Appl. Phys. 103, 074112 (2008). Extensive research efforts by many groups have investigated the causes of and solutions to these issues. Several reports indicate that zirconium, tantalum, titanium oxide, and aluminum oxide adhesion layers are superior to the more common titanium in terms of lower diffusivity through the platinum layer, limited reaction with the underlying substrate, and a lack of polymorphic phase changes over the desired processing temperature range resulting in fewer mechanical defects (delamination, roughening, and hillocking). See H. N. Al-Shareef et al., J. Mater. Res. 12, 347 (1997); S. H. Kim et al., Appl. Phys. Lett. 76, 496 (2000); T. Maeder et al., Jpn. J. Appl. Phys. 1 37, 2007 (1998); G. R. Fox and K. Suu, U.S. Pat. No. 6,682,772; K. Sreenivas et al., J. Appl. Phys. 75, 232 (1994); and S. Halder et al., Appl. Phys. A-Mater. Sci. Process. 87, 705 (2007). These advances notwithstanding, titanium adhesion layers remain the most widespread.
Therefore, a need remains for an adhesion layer that can be used to metallize a silicon substrate and is compatible with high temperature processing.